"Ferrite rod antennas are quite tricky to get
working properly - you need the highest
permeability rod possible, _but_ the loss
must be low at your operational frequency.
Most medium wave rods will work OK at
2MHz, but probably not at 3 or 4."
I have been looking for an opening into this
discussion and the mention of ferrite rods and
DFing at SW frequencies has finally drawn me in.
I have had EXCELLENT results using an old Sears
Model 2278 4-band AM/SW/FM "portable" to do
DF operations on 80 meters - for example, using this
receiver, over the last 2 years I have been able to do
quick and accurate DFing (using the null techniques
described previously) to two different distribution-level
line noise sources that had just 'popped up' out
of nowhere.
This receiver also incorporates a signal strength
meter - INDISPENSIBLE IMO when determining the
actual source/specific pole when 'close-in' (read that
as "when measuring the signal strength exhibited
near the ground wire which comes down the pole").
These two different sources ('power factor correction'
capacitor banks) were on the order of 1/2 to 1 mile
away and were affecting several of us hams here in
the area - one gentleman had even taken his 2 Meter
gear mobile with a yagi and did not have the success
I had using this 1968 vintage battery-powered Sears
AM/SW radio. (A subsequent call to TXU remedied
the problem - a mention that the "timed control box"
mounted near the base of the pole was unlocked
expeditied attention to the problem as well.)
This particular receiver utilizes a good-sized ferrite
rod as the antenna on MW and SW 1 (2 - 6 MHz).
When tuning for a 'noise source' definite variances
in signal strengths can be found for most noise
sources - using a broad-band tunable receiver one
almost has a portable, battery-operated spectrum
analyzer (albeit an un-cal'd analyzer) at one's
disposal!
The uppper band (SW 2, 6 - 18 MHz) seems to use
the whip antenna only, and I have not found that I
can do 'DFing' on this band using any exhibited
directionality.
I have also used this receiver when DFing those
pesky switching-power-supply based gel-cell battery
chargers imported from China (chargers which had
NO Part 15 label; nor was there included in the user
manual the required Part 15 wording or text) as
imported by these people:
http://www.terminatorscooter.com/
DFing this little noise maker required literally 'walking
the streets' (and sidewalks; getting as physically close
to houses as legally practical) in the area (like walking
plus or minus one street north and south of where the
source actually turned out to be) and mentally
assimilating the measured field strengths; underground
electrical utilities and being close to the source as well
as the relatively low frequencies involved (DFing in the
2-6 MHz band) made for tough identification 'down to
the house' in this case.
The key to identifying this noise maker was striking up
a conversation with the homeowner (he and his kids had
come outside!) where the signal strength was strongest -
turns out he was having TVI problems and did't know his
scooter chargers were the culprits!
Jim P / WB5WPA /
----- Original Message -----
From: "Roger Parsons" <ve3zi@yahoo.com>
To: <rfi@contesting.com>
Sent: Monday, February 14, 2005 8:31 AM
Subject: [RFI] What antennas are good for HF DFing?
> You can use either a loop or a ferrite rod antenna.
> The performance will be quite similar. The sensitivity
> will be greatly improved if you use a tuned loop, and
> that may or may not matter in your particular
> situation. It probably will matter, as you listen for
> a NULL rather than maximum - the null will be a few
> degrees wide and the maximum perhaps 90 degrees. It is
> important that the antenna be constructed in as
> symmetrical way as possible, and kept a reasonable
> distance (a few cms) away from any metallic objects,
> such as the receiver. If this is not done, the null
> positions will probably not be 180 degrees apart, and
> shielded loops are somewhat better in this respect
> (although probably not worth the extra effort in
> construction). (There must be a break in the shield,
> for a loop directly opposite the feed point, and for a
> ferrite rod use a slotted tube.)
>
> Ferrite rod antennas are quite tricky to get working
> properly - you need the highest permeability rod
> possible, _but_ the loss must be low at your
> operational frequency. Most medium wave rods will work
> OK at 2MHz, but probably not at 3 or 4. To increase
> sensitivity you can use several rods, _but_ just
> strapping them in parallel or lengthwise will only
> produce a miniscule improvement. You can get a 3dB
> improvement for 2 rods by spacing them parallel about
> 5cm and splitting the tuned and coupling winding
> between the two rods with a single tuning capacitor.
> Probably not worth the effort, as a 30cm square loop
> will be just as good and is a lot easier to make.
>
> Your bearings will have a 180 degree ambiguity, which
> makes life somewhat tricky. This can be resolved by
> adding a sense antenna. This is an omnidirectional
> antenna (such as a whip), the output of which is
> combined with the loop. If the outputs from each
> antenna are the same, you will then get a broad
> maximum in one direction and a broad minimum in the
> other - broadside to the rod, or in line with the
> loop.
>
> The technique is to obtain a bearing first, without
> the sense antenna in use - this should be within a few
> degrees - then turn the antenna at 90/270 degrees and
> see from which direction the signal is arriving.
> Obviously you need to practice first on known signals!
>
> All of the above works wonderfully if
> (1) The signal being DFed is arriving solely by ground
> wave. This will probably be true, but is a good reason
> to only do your tests during the day when sky wave
> propagation on mf is minimal.
> (2) There are no extraneous conductors, such as power
> lines around. The latter is most unlikely to be the
> case if you are trying to track down noise sources.
> Power lines can carry mf signals for tens of
> kilometres with low loss (BPL...), and because there
> are standing waves present, and because the poles
> often have a nice grounded vertical (the ground
> connection), there will probably be multiple maxima
> and minima. The solution is to get well away from the
> power line to take your initial bearings - several
> wavelengths. This can be very difficult of course if
> the signal can only be heard close to the power line.
>
> The professionals all advise to use the highest
> possible frequency at which the signal can be heard -
> there is most unlikely to be ambiguity DFing on
> 150MHz. mf DF can be done, but it is very, very easy
> to literally go down the wrong garden path!
>
> GL and 73
> Roger VE3ZI
>
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